1. Field of the Invention
The present invention relates to a shift valve of a hydraulic control system for an automatic transmission of a vehicle, and more particularly to the shift valve with a simple construction, which ensures safety by maintaining a formal shifting operation when an electronic control system is out of order.
2. Description of the Related Art
A conventional automatic transmission for a vehicle has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter, which includes a hydraulically actuated friction member for selecting one of a plurality of gear stages of the transmission gear mechanism in accordance with vehicle operating conditions.
A hydraulic control system pressurized by a fluid pump provides working pressure required to operate the friction members and control valves.
The commonly used automatic transmission for a vehicle has the torque converter which generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine-driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the hydraulic fluid from the turbine runner to a direction where fluid flow does not disturb a rotation of the pump impeller when fluid flows into the pump impeller.
An automatic shift is made by an operation of the friction members such as clutches or a kick-down brake at each shift change. Also, a manual valve, a port of which is converted by selecting a position of a selector lever, is designed to be supplied with fluid from a fluid pump and to supply the fluid to a shift control valve. In a 4-speed automatic transmission, the shift control valve has an opening port changed by an electronic control system.
FIG. 5 is a sectional side view for depicting an operation state of a conventional shift control valve and manual valve, where two solenoid valves 1 and 2 operated according to a signal of a transmission control unit TCU, are connected with a manual valve 3, via a line 4 and a shift valve 5, and is designed to regulate line pressure.
Hydraulic pressure flowing into the shift control valve 5 through the line 4 is designed to work on a first land 6 and a second land 7 of a valve spool S, and two plugs 8 and 9 are disposed at both sides of the valve spool S to selectively change the line pressure according to the operation of the solenoid valves 1 and 2.
With respect to the above explained shift control valve 5, in a first speed stage, since both solenoid valves 1 and 2 are turned ON by the TCU, the line pressure supplied from the manual valve 3 is exhausted to exhaust ports EX via the shift control valve 5 and line 4.
Accordingly, the hydraulic pressure does not work on ports 5a and 5b of the shift control valve 5, and the line pressure from the manual valve 3 works on the first and the second lands 6 and 7, at this point, since a section area of the first land 6 is larger than that of the second land 7, the valve spool S is moved to the left and maintains the state of pushing the plug 8.
By the above described operation, the hydraulic pressure from a port 3a flows into a rear clutch (not shown) via a line C, and operates the rear clutch thereby realizing the first speed stage.
In a second speed stage, since only the solenoid valve 2 is turned ON by the TCU, the hydraulic pressure working on the port 5b is exhausted, the solenoid pressure works on the plug 8 and 9 via the port 5a thereby pushing the plug 8 rightward. The plug 8 is stopped by a stopper 8a.
At this point, since the plug 8 pushes the valve spool S rightward, the position of the land 7 is changed so that the line 4 and a port 5c communicate with each other, and the hydraulic pressure works on first-second speed shift valve (not shown) and an end clutch (not shown) thereby realizing the second speed stage.
In a third speed stage, since both of the two solenoid valves 1 and 2 are turned "OFF" by the TCU, the hydraulic pressure works on the ports 5a and 5b, and moves the valve spool S to a location of a stopper 9a.
According to such an operation, since the land 7 is designed to be located on the right side of a port 5d, the hydraulic pressure from the line 4 works on second-third/fourth-third speed shift valves (not shown) and the end clutch (not shown) whereby the third speed stage is realized.
Further, in a fourth speed stage, since only the solenoid valve 1 is turned ON by the TCU, the solenoid pressure working on the port 5a is exhausted, and in the case where the solenoid pressure working on the port 5b is higher than an elastic force of a spring which elastically supports the plug 9, the solenoid pressure moves the valve spool S and plug 9 to the right, locating the land 7 on the right side of a port 5e, so that the hydraulic pressure from the line 4 is supplied to the rear clutch (not shown) via the port 5e to achieve the fourth speed stage.
The above described conventional shift valve has some disadvantages that complicate construction of the electronic control due to using two solenoid valves which are respectively controlled by the electronic control system.
Additionally, in the case where the electronic control system is out of order, the solenoid valves 1 and 2 do not operate, so that the hydraulic pressure works on the ports 5a and 5b. Accordingly, a driver always has to drive a vehicle at the third speed stage.
As described above, if the vehicle is driven only at the third speed stage without the first and the second speed stages, an engine brake effect cannot be obtained in the vehicle, so that a safety problem occurs.
Further, U.S. Pat. No. 4,391,166 discloses a control system wherein a valve prevents up-shifting from a low speed stage when the electronic control system is out of order and transmission is set in a first speed stage.